The invention relates generally to LED structures and more particularly to the active region of an LED.
Light emitting diodes (LEDs) are widely accepted in many applications that require low power consumption, small dimensions, and high reliability. However, the use of LEDs in new applications is limited by their external quantum efficiency or their brightness. Therefore, many attempts have been made to improve the brightness of LEDs through various design changes. For example, improvements in LED brightness have been achieved by using multi-well active layer devices in which multiple light emitting active layers are included in the LED active region. Additional light output gains have been observed by decreasing the thickness of the individual light emitting active layers, and in the extreme case, the thickness of the individual active layers is reduced to the point where quantum confinement effects are observed (i.e., discrete or quantized energy states occur within the active layers). In such a case, the active layer thicknesses are said to have been reduced below the quantum thickness and such devices are said to operate in the quantum regime or quantum size regime. This quantum thickness depends on certain material parameters such as the electron or hole effective mass, and is therefore different for different materials. For AlGalnP LEDs, the quantum thickness is about 100 Angstroms, while for AlGalnN LEDs, the quantum thickness is about 60 Angstroms. In the context of this disclosure, we define the aforementioned LEDs as multi-well (MW) LEDs, regardless of individual active layer thickness, i.e., regardless of whether the individual active layers are thinner than the quantum thickness or thicker than the quantum thickness. Examples of such MW active layer LEDs and laser diodes are provided in U.S. Pat. No. 4,318,059 to Lang et al., U.S. Pat. No. 5,410,159 to Sugawara et al. and U.S. Pat. No. 5,661,742 to Huang et al.
In contrast to the MW LEDs discussed above, LEDs having a single active layer will be referred to either as double heterostructure (DH) LEDs, or as single quantum well (SQW) LEDs, depending on whether the individual active layer thickness values are greater than or less than the quantum thickness, respectively.
A conventional MW LED is schematically illustrated in FIG. 1. The LED 10 includes a substrate 12 of first conductivity type, a lower confining layer 14 of first conductivity type, the active region 16 which may be of first conductivity type, may be undoped, or may be of second conductivity type, an upper confining layer 18 of second conductivity type, and an optional window layer 20 of second conductivity type. The active region includes two or more thin active layers 22 that are separated from each other by one or more barrier layers 24. Although the active region is shown to include four active layers, the number of active layers can be anywhere from two to forty or more. In the most common configuration, the lower confining layer is made of an n-type semiconductor material, while the upper confining layer is made of a p-type semiconductor material. In this case, the n-type lower confining layer is electrically connected to an n-type ohmic contact 26 via the substrate 12, and the p-type upper confining layer 18 or optional p-type window layer 20 is electrically connected to a p-type ohmic contact 28. (It is also possible to grow or bond or otherwise attach the LED to a p-type substrate or other material such as metal, glass, etc., such that the lower confining layer is p-type and the upper confining layer and optional window layer are n-type. Since the most common LED configuration includes an n-type substrate, we use this case as an example here. Hence, in these examples, the first conductivity type is n-type, and the second conductivity type is p-type.)
When a potential is applied to the ohmic contacts 26 and 28 , electrons are injected into the active region 16 from the n-type lower confining layer 14 and holes are injected into the active region from the p-type upper confining layer 18. The radiative recombination of electrons and holes within the active layers 22 generates light. However, if the recombination occurs within a layer other than one of the active layers, such as the lower confining layer, the upper confining layer, or a barrier layer within the active region, no light is generated. Thus, it is desirable to increase the probability that the electrons and holes recombine within the active layers, as opposed to recombining within some other layer of the device. The multiple wells formed by the active layers 22 of the LED 10 increase the radiative recombination probability by allowing holes or electrons that did not recombine in one of the active layers a chance to recombine in another active layer. The increase in radiative recombination of electrons and holes within the active layers of the LED equates to an increase in the light output of the LED.
Although light output gains can be realized by implementing a multi-well structure, additional light output gains are desired to achieve more widespread use of LEDs. A concern with conventional LED designs is that light is not emitted equally from all wells, and in some extreme cases, especially for AlGalnN devices (as will be illustrated in FIG. 7), most of the light is emitted only from one or two wells in the structure. Thus, some of the wells in a conventional LED do not contribute effectively to the brightness of the LED. This problem is compounded in a transparent substrate LED in that some fraction of the light that is generated within one active layer may be absorbed in another active layer of the active region. Thus, an active layer that does not contribute efficiently to light generation limits the light output of the device in two ways. First of course, it does not generate light efficiently. Second, it may absorb some fraction of the light generated by other active layers within the active region.
In light of the above concern, what is needed is a method for increasing the light output or light generation efficiency of each well in a multi-well LED.
A light emitting device and a method of improving the light output of the device utilize a chirped multi-well active region to increase the probability of radiative recombination of electrons and holes within the light emitting active layers of the active region by altering the distribution of electrons and holes within the light emitting active layers of the active region (i.e., across the active region).
In an exemplary embodiment, the LED is an AlGaInP LED that includes a substrate of first conductivity type, an optional distributed Bragg reflector layer of first conductivity type, a lower confining layer of first conductivity type, an optional lower set-back layer of first conductivity type, the chirped multi-well active region which may be of first conductivity type, may be undoped, or may be of second conductivity type, an optional upper set-back layer of second conductivity type, an upper confining layer of second conductivity type, and an optional window layer of second conductivity type. The substrate is made of a semiconductor material, such as GaAs or GaP. In a preferred embodiment, the lower confining layer is composed of an n-type (AlxGa1xe2x88x92x)yIn1xe2x88x92yP material, where xxe2x89xa70.6 and y=0.5xc2x10.1, while the upper confining layer is composed of a p-type (AlxGa1xe2x88x92x)yIn1xe2x88x92yP material, where xxe2x89xa70.6 and y=0.5xc2x10.1. The optional upper set-back layer is formed of an undoped (AlxGa1xe2x88x92x)yIn1xe2x88x92yP material, where xxe2x89xa70.6 and y=0.5xc2x10.1. The optional upper set-back layer may be used to help control the diffusion of p-type dopants from the upper confining layer into the active region during high temperature processing steps. The optional lower set-back layer may also be formed of an undoped or n-type (AlxGa1xe2x88x92x)yIn1xe2x88x92yP material, where xxe2x89xa70.6 and y=0.5xc2x10.1. The optional upper and lower set-back layers also generally have an aluminum composition, x, which is less than or equal to the aluminum composition of the upper and lower confining layers, although this is not necessarily the case. In the preferred embodiment discussed above, the first conductivity type is n-type and the second conductivity type is p-type. Since this is the most common LED configuration, we use this configuration as an example throughout this disclosure. It is also possible, however, to form the LED where the first conductivity type is p-type and the second conductivity type is n-type. The invention disclosed herein will work in either configuration.
The chirped multi-well active region of the LED includes N light emitting active layers and Nxe2x88x921 barrier layers, where N is an integer greater than one. In this embodiment, the light emitting active layers and the barrier layers are made of (AlxGa1xe2x88x92x)yIn1xe2x88x92yP, where 0xe2x89xa6xxe2x89xa61 and y=0.5xc2x10.1. The term xe2x80x9cchirpxe2x80x9d refers to non-uniform configuring or asymmetric configuring of similar layers with respect to their thickness and/or composition.
The chirped multi-well active region LED thus consists of a multi-well active region where the active layers and/or barrier layers are dissimilar in terms of their thickness and/or composition. Note that the differences in thickness and/or composition of the active layers and/or barrier layers are small enough that each active layer emits substantially the same color light, resulting in a highly monochromatic LED. In one embodiment of the chirped active region LED, the individual active layers within the active region are of non-equal thickness. In another embodiment of the chirped active region LED, the individual active layers are of equal thickness, but are of unequal composition. In yet another embodiment, the individual active layers are of unequal thickness and unequal composition. In yet another embodiment, the active layers are of equal thickness and equal composition, but the barrier layers between the active layers are of unequal composition, or unequal thickness, or of both unequal composition and unequal thickness. In yet another embodiment, a combination of non-equal thickness and/or composition of various barrier layers and/or active layers may be used within the active region.
The chirped multi-well active region design can be implemented in AlGalnP LEDs as described here, or in other III-V material LEDs, II-VI material LEDs, polymer or organic LEDs, and in other light generating devices, such as laser diodes and optical amplifiers, to improve the light output of that device.